DIGIS.TXT         DIGIPEATER SET UP IN APRS
Document version: 8.6.7  24 July 2004       
Prior versions:   26 Sept 2003. (prior version was 15 Sept 2001)
Author(s):        Bob Bruninga, WB4APR <bruninga@nadn.navy.mil>
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In July 2004, much of the USER PATH information was split from this
file and placed in PATHS.TXT  and only the DIGIPEATER information
needed by DIGI sysops was retained in this file.  But SYSOPS must
be fully familiair with PATHS.TXT also.

This file originated in 1994 to describe the detail objectives of the
unique generic APRS approach to digipeating.  It is still current and is
a MUST READ for anyone who seriously uses APRS.  Further, users must also
read the on-line NEW N-N PARADIGM concept that tells what is WRONG with
many area digipeater implementations of APRS on 144.39 and how many local
nets are totally saturated and what must be done to fix it.

   google for "fix14439.html"

Digipeaters are the most important APRS asset, our lifeline if used 
properly, but a source of QRM and inconsistencies if not.  First, here
is what an APRSdigi using typical KPC-3+ firmware is expected to do:

1) Digipeat the aliases of RELAY, WIDE, TRACE, SS with callsign
   substitution.  Note that under the new n-N paradigm TRACE is no
   longer required and my be substituded with other local options
   such as another adjacent state, SAR, WX, ARES, RACES, whatever...

2) Support two of the following n-N routing strategies using either the
   UIFLOOD or UITRACE settings in the TNC:
     WIDEn-N in vast areas with sparse users
     LINKn-N in linear paths (along coasts, Interstates, around cities)
     LANn-N  in specific geographical areas

3) Use of UIFLOOD must have the default of NOID so that the digi does
   not overwrite original path data in every packet with its own ID.
   Do NOT enable ID!

4) Identify itself with a position packet once every 10 minutes LOCALLY
   and DIRECT only.  Then every 20 to 30 minutes via one hop, and no
   more than once every hour or more for 2 hops or more.  A special
   technique using the LText, LTPath and BLT functions is used to
   implement this complex technique.

5) The POSITION packet should indicate the proper ICON and overlay
   character and in its comment field identify any special features.

6) The unused BText may be used at a low rate LOCALLY and DIRECT to
   also put one special local object on the map periodically.

DIGIPEATER LOCAL NETWORK SIZE:
As noted in PATHS.TXT, a local network cannot possibly support more than
about 50 or so users on the 144.39 channel no matter how many digis it
takes to do it.  Users and DIGI owners must understand that they are
only trying to serve RELIABLY about 50 or so users on the channel.  In
big cities, this might only be a 15 mile radius needing only one DIGI.
In other sparse areas, it might be 150 miles and need 9 digis to cover
the typical 50 or so users.  This range is called the ALOHA circle and
is easily determined ANYWHERE.  Users and digi owners then need to
ensure that everyone operates with a path only sufficient to hit only
that area, otherwise they are just causing QRM to everyone else.

FEWER HOPS:  This table from PATHS.TXT is repeated here because it shows
the futility of long multi-hop paths.  It is just not worth it and is
not the design of APRS to go beyond its own local network of 0 or so
users.  If users want a bigger picture, they should go to the internet!

HOPS  PROBABILITY  #COPIES COMMENTS
----  -----------  ------- --------------------------------------
 1        50%         1    For local ops & special events
 2        25%         5    Routine ops
 3        12%        13    extended ops
 4         6%        25    statewide ops Heavy QRM
 5         3%        41    Nothing gets through.  Too much QRM
 6         1%        61    Useless AND totally boggs down network


THE NEW n-N PARADIGM:
---------------------

in 2003, we realized that APRS had grown by orders of magnitude above
its self-replicating beginnings, and that the original goals of local
flooding of all packets to all nearby digipeaters was killing the
network.  Though WIDE5-5 paths were desired early on, by 2003, they
were killers.  In most areas anything beyond WIDE2-2 was unnecessary.
Thus, WIDEn-N is now a BAD thing because it allows too easy flooding.

To combat this, the new N-N PARADIGM was defined to give local SYSOPS
several tools they could use to regain control of the traffic in their
local area without significantly having  to modify user behavior:

1) Simply disable WIDEn-N in big Cities.  This prevents distant long hop
   n-N packets from entering local LANs.  Or at those digis, have the
   n-N packets shifted to a UHF channel so they can still be heard
   if needed.

2) Use the UIFLOOD n-N algorithm only for linear chains of digipeaters.
   These LINKn-N linear chains only generate 2 * N copies of any packet
   instead of the 4^N flooding of the old WIDEn-N operations.

3) Change UIFLOOD n-N to LANn-N where there are only X number of digis
   in any given LAN name.  Thus even LAN7-7 still only generates X
   copies where X is a manageable number for the desired LAN

4) Encourage shorter paths.  Instead of asking locals to use WIDE,WIDE2-2
   so that the initial digi is identified, change the DIGI from UIFLOOD
   WIDE to UITRACE WIDE.  Then locals can simply use WIDE3-3 and their
   packets will always be traced, and the paths are short.

LINKn-N CONVENTIONS:

Anywhere that there is a linear alignment of digis that serve a common
purpose and set of users is an ideal location for the LINKn-N construct.
Examples are COASTn-N along the East Coast serving boaters.  Or 81LNKn-N
along the interstate 81 corredor, or 95LNKn-N along interstate 95.

ALT-CHANNEL INPUTS:

Another very powerful local technique involves adding an additional
local digipeater input channel dedicated to only WX stations and FIXED
stations.  Since they only transmit at low duty cycles, there can be
dozens of them sharing this alternate input with little chance of
collision, yet once their packet is heard on the alt channel and then
digipeated on the main channel, their packet is guaranteed not to
collide because the DIGI first listens before transmitting and it can
HEAR everything.  A good frequency (if available in your area, might
be 144.99 since it allows users to use the standard +600 KHz TX offset
from 144.39.  These fixed stations still always listen on 144.39 so that
they still hear everything.  GOOGLE:  alt-channel.html


DEDICATED WIDE AREA APRS DIGIPEATER SET UP

The BEST digi used to be a new Kantronics KPC-3 or 3+.  Next best was  the
PacComm ROM in any TAPR-2 CLONE TNC.  But these days there are many better
options.  DIGI_NED is simply software that runs on any old 286PC or clone
via any old TNC in KISS mode.  UIDIGI is a ROM system that can plug into
ANY old TAPR-2 compatible TNC (there are hundreds of thousands of these
out there gathering dust)...  Since I cannot address all of the various
settings in every type of TNC, here are the settings for the KPC-3:

   MYCall W3XYZ-x                       
   UIDIGI ON RELAY, WIDE, SS, ???       <= these do callsign substitution.
   UIFLOOD LINK,28,NOID                 <= Use these for WIDEn-N, LINKn-N
   UITRACE WIDE,28                      <= or LANn-N's depending on needs.
   HID OFF
   LT 1 !DDMM.mmN/DDDMM.mmW#PHG5360/A=003456/comments
   LT 2 !DDMM.mmN/DDDMM.mmW#PHG5360/A=003456/comments)
   LT 3 !DDMM.mmN/DDDMM.mmW#PHG5360/A=003456/comments)
   LT 4 !DDMM.mmN/DDDMM.mmW#PHG5360/A=003456/comments)
   LTP 1 APRS
   LTP 2 APRS VIA WIDE
   LTP 3 APRS VIA WIDE2-2
   LTP 4 APRS VIA TRACE3-3
   BLT 1 E 00:20:00 At 00:00:00  Sends local posit locally every 20 mins
   BLT 2 E 00:40:00 At 00:10:00  Sends posit out 1 hops every 40 minutes
   BLT 3 E 01:20:00 At 00:30:00  Sends a 2 hop path once every 80 mins
   BLT 4 E 02:40:00 At 01:10:00  Sends a 3 hop path once every 160 Mins

   The result is one local packet every 10 minutes and one area packet
   every 30 minutes which covers the two APRS standard net-cycle times.
   Note that HID MUST BE OFF.  See HID.TXT.

POSIT TEXT:  The posit text above is standard APRS posit as follows:

   !                    means it is a fixed, non moving posit
   DDMM.mmN/DDDMM.mmW   is LAT/LONG in degrees and minutes
   PHGphgd              where p is power as the SQRT of P 
                              h is log2(HAAT/10)
                              G is gain in dB
                              d is directivity in deg/45
   #                    means it is a digipeater
   /                    The separator between the LAT/LONG
                        should be:  / for WIDE or RELAYS
                                    \ for WIDE-RELAYS
                                    T for TRACE digis
                                    N for WIDEn-n digis
   /A=xxxxxx            is altitude in feet for 3D

You can see by the integers in the POWER-HEIGHT-GAIN (PHG) string, there 
are only 9 plus 0 possible values for each of these fields as follows:

  DIGITS   0  1  2   3   4   5   6    7    8    9  as used in the PHG field
  -------------------------------------------------------------------------
  POWER    0, 1, 4,  9, 16, 25, 36,  49,  64,  81  watts  SQR(P)
  HEIGHT  10,20,40, 80,160,320,640,1280,2560,5120  feet   LOG2(H/10)
  GAIN     0, 1, 2,  3,  4,  5,  6,   7,   8,   9  dB
  DIR   OMNI,NE, E, SE,  S, SW,  W,  NW,   N,   .  This offsets the range
                                                   circle in the indicated
                                                   direction
           
HEIGHT ABOVE AVERAGE TERRAIN:  Going out 10 miles in all directions, write
down the elevation every mile or so.  Average all of these points and
compare your elevation to the average.  You may be at 2000 feet above
sea level and have a 150 foot tower, but if the ground around you is at
2200 feet, then your HAAT is -50 feet!!!  Be honest!  Your circle should
go no further than the distance to which you can reliably copy an HT!  
Even though you have an OMNI antenna, if the terrain favors a certain 
direction, then put that in for your directivity.  The PHG equation
assumes nominal stations.  It is not good for HAAT's in the thousands
of feet.  For those digis, chose a value for H or G that gives a good
reliable PHG circle that matches actual coverage.

SYSOP REMINDERS!

WIDEn-n ID/NOID FUNCTION:  UIFLOOD must be set with NOID (the default
or this digi will obliterate prior path information on every packet.
This is just wrong for an APRS network where the most important
info in the path is where it entered the newtork.  For more on this
topic and the damage that the "ID" setting does, see:

  GOOGLE:  id-noid.txt

Also be sure to see the entire new n-N paradigm and all the other
problems we need to fix on the global APRS network (144.39 in
north america).

  GOOGLE:   fix14439.html

Be creative, we have only begun to tap the potential of LINKn-N
and LANn-N networks to cut down on the QRM from user abuse of
WIDE5-5 and longer paths in urban areas.  There is so much we CAN
do.

de WB4APR, Bob
